This invention generally relates to a mold and method for forming small diameter balloons suitable for dilatation catheters such as are used in angioplasty procedures.
In typical percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter having a preformed distal tip is percutaneously introduced into the cardiovascular system of a patient through the brachial or femoral arteries and advanced therein until the distal tip thereof is in the ostium of the desired coronary artery. A guidewire and a dilatation catheter having a balloon on the distal end thereof are introduced through the guiding catheter with the guidewire slidably disposed within an inner lumen of the dilatation catheter. The guidewire is first advanced out of the distal tip of the guiding catheter and into the patient's coronary vasculature until the distal end of the guidewire crosses the lesion to be dilated. Then the dilatation catheter is advanced out the distal end of the guiding catheter over the previously introduced guidewire until the dilatation balloon is properly positioned across the lesion. Once in position across the lesion, the flexible, relatively inelastic balloon is inflated to a predetermined size with radiopaque liquid at relatively high pressures (e.g., greater than about 4 atmospheres) to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall to thereby dilate the lumen of the artery. The balloon is then deflated so that the dilatation catheter can be removed and blood flow resumed through the dilated artery.
Further details of angioplasty procedures and the devices used in such procedures can be found in U.S. Pat. No. 4,323,071 (Simpson-Robert); U.S. Pat. No. 4,332,254 (Lundquist); U.S. Pat. No. 4,439,185 (Lundquist); U.S. Pat. No. 4,168,224 (Enzmann et al.) U.S. Pat. No. 4,516,972 (Samson); U.S. Pat. No. 4,538,622 (Samson et al.); U.S. Pat. No. 4,554,929 (Samson et al.); and U.S. Pat. No. 4,616,652 (Simpson) which are hereby incorporated in their entirety by reference thereto.
Steerable dilatation catheters with built-in or fixed guidewires or guiding elements are frequently used because such catheters generally have smaller deflated profiles than conventional dilatation catheters with movable guidewires with equivalent balloon size. The lower deflated profile of these catheters allows them to cross tighter lesions and to be advanced much deeper into the patient's coronary anatomy. Further details of low-profile steerable dilatation catheters may be found in U.S. Pat. No. 4,582,181 (Samson); U.S. Pat. No. 4,619,263 (Frisbie et al.); U.S. Pat. No. 4,641,654 (Samson et al.); U.S. Pat. No. 4,664,113 (Frisbie et al.), U.S. Pat. No. 4,771,778 (Mar) and U.S. Pat. No. 4,793,350 (Mar et al.) which are hereby incorporated in their entirety by reference thereto.
Progress in the development of angioplasty catheters has included significant reductions in the deflated profiles of such catheters which allow them to be advanced through tighter stenoses and much deeper into the patient's coronary anatomy. The use of high-strength materials for the dilatation balloon and other catheter components have aided in this progress by allowing much thinner balloon walls. Biaxially oriented, high-strength plastics such as polyethylene terephthalate (PET) have been found to be particularly effective in this regard. See for example, U.S. Pat. No. 4,456,000 (Schjeldahl et al.).
The prior art dilatation balloons made of polyethylene and the like were usually heat set after forming with the interior of the balloon under a vacuum so that when the balloon is subsequently subjected to a vacuum prior to inserting or removing the dilatation catheter the "wings" of the balloon would curve about an inner member of the catheter assembly. This greatly reduced the effective profile of the balloon and allowed it to be more readily advanced into and withdrawn from the patient's arterial system.
However, many high-strength plastic materials, particularly polyesters such as PET, do not readily heat set so that the wings of the balloon cannot be preshaped to curve around an inner member when the interior of the balloon is subjected to a vacuum. When a balloon made from such polyester material is subjected to a vacuum, the wings thereof generally extend radially away from the inner member forming a substantial profile in at least one plane which can interfere with the advancement and withdrawal of the balloon. This minimizes one of the main reasons for using the high-strength material, namely, reduced deflated profiles. Moreover, the edge of the wings of the deflated balloon may be sharp enough to damage the interior lining of the artery.
In copending application Ser. No. 397,985 filed Aug. 23, 1989 a dilatation catheter is described which has a balloon formed into a prism-like shape with a polygonal transverse cross-section. Preferably, the polygonal cross section has from three to six sides, the triangular and square cross sections being preferred. When the prism-like balloon is inflated to the pressures normally encountered in angioplasty procedures, it expands to a conventional, generally circular transverse cross section to effectively dilate a stenotic region of a patient's artery. When the interior of the balloon of the invention is subjected to a vacuum, the configuration formed has the same number of wings as sides in the polygonal shape, which PG,5 greatly reduces the span thereof and decreases the effective deflated profile of the balloon. Additionally, the balloon so formed will consistently collapse to the same deflated configuration having three or more wings when a vacuum is applied to the balloon interior.
Prior techniques for the manufacture of the prism shaped balloon included extruding the desired polymer resin into a tubular form, then inflating the tube at elevated temperatures and pressures in a heated mold having an interior surface of the desired shape. The high temperatures allow the pressurized tubing within the mold to expand and take the form thereof. The molds were made in two or more pieces and required very accurate machining of the interior surface thereof in order to form acceptible balloons for angioplasty catheters. However, notwithstanding how accurately the mold was machined, parting lines were molded into the balloon surface at the junctions of the mold sections. These parting lines on the surface of the working portion of the balloon created areas of weakness and also prevented the uniform expansion of the balloon.
What has been needed and heretofore unavailable is a balloon mold and a method of operating a mold which does not generate parting lines in the surface of the final balloon product. The present invention satisfies this need.